In most instances, commercial lighting fixtures are employed in suspended ceiling systems wherein a framework of inverted "T" bars or the like are utilized to support the entire ceiling system as well as the lighting fixtures. In some ceiling systems, the bottom face of the "T" bar is left exposed to provide a grid-like appearance to the ceiling system. In those types of installations, simple lay-in fixtures are employed which are supported directly by the "T" bar flanges, as are the acoustical ceiling tiles. In other systems, it is preferred that the bottom face of the "T" bar ceiling support system be hidden in order to avoid the grid appearance provided by the exposed "T" bar system. In the latter case, the acoustical ceiling tiles generally have a horizontal slot in their side edges which permits the acoustical tile, when placed on the "T" bars, to hide half of the "T" bar flange face while the adjacent ceiling tile will hide the other half of that flange face. Where lighting fixtures are employed in this type of ceiling system, it is generally required that after the luminaire is mounted within the ceiling system on the supporting "T" bar framework, an exterior flange must then be secured to the luminaire in order to hide the flanges of the "T" bar adjacent the opening in which the lighting fixture is mounted.
Another method sometimes employed to hide the ceiling grid system is to employ separate steel or vinyl holding strips which are mounted directly to the flange of the "T" bar adjacent the luminaire to provide the framework for the luminaire as well as hide the grid framework. Another method of providing this shielding or concealing of the "T" bar framework is by employing a luminaire with a complex door frame which provides both the luminaire door with its appearance frame and also carries both the air return structure and a second outer frame, also secured to the door frame which extends outwardly and shields the "T" bar framework from view.
Each of the foregoing systems provides either a complex system from the standpoint of mounting or an expensive system from the standpoint of luminaire manufacture.